In the Fischer-Tropsch conversion of synthesis gas to hydrocarbons, cobalt is the preferred active metal when the feed to the syngas unit is natural gas. This choice is essentially based on the low shift activity of Co, that otherwise would convert some of the CO in the syngas to CO2 and hydrogen, thereby losing some of the carbon in the feed.
Other known FT active metals are iron, ruthenium and nickel. Iron is frequently used particularly when the syngas is based on coal, as the inherent shift activity is needed to adjust the CO/H2 ratio to the desired ratio around 2. Ruthenium is prohibited due to excessive cost, whereas nickel is rejected due to high selectivity to methane, thus giving rise to backformation of the gas feed. It is well known that nickel catalysts are used for methanation, removing traces of residual CO in the feed for ammonia synthesis.
Normally the active FT-metal is dispersed on a solid support. The support can be alumina, titania or silica, as well as a variety of other oxides and mixed oxides, and the support can be chemically stabilized or treated in a number of ways. Of particular relevance is high temperature treatment of alumina giving a catalyst with high content of alpha-alumina, thereby increasing the selectivity to higher hydrocarbons (C5+), as disclosed in WO 02/47816 A1 (Statoil).
Preparation of the Catalyst can Involve Impregnation on the Support by a Selected technique, or co-precipitation with other ingredients in addition to the cobalt precursor. Subsequent forming to the desired shape can also be part of the procedure. Further, the catalyst preparation normally contains steps like drying, calcination and reduction to give the active catalyst. During preparation, a number of other elements or compounds often are added. These can be denoted as modifying agents, structural stability promoters, or promoters intended to increase selectivity, activity, stability or regeneration performance of the catalysts. Some modifiers or promoters frequently investigated are thoria, zirconia, manganese, alkali metals, lanthanum oxide or a mixture of lanthanides, rhenium, ruthenium and platinum.
A number of alternative impregnation procedures are known in the art which use alternative solvents and chemicals, however, in the present invention, the examples involve aqueous incipient wetness with solutions of cobalt nitrate (Co(NO3)2. 6H2O) and possibly perrhenic acid (HReO4) or ammonium perrhenate. Alternatives include using cobalt acetate(s), cobalt halide(s), cobalt carbonyl(s), cobalt oxalate(s), cobalt phosphate(s), organic cobalt compounds, ammonium perrhenate, rhenium halide(s), rhenium carbonyl(s), industrial metal salt solutions, organic solvents, etc.
Incipient wetness implies that the metal containing solution is mixed with the dry support until the pores are filled. The definition of the end point of this method may vary somewhat from laboratory to laboratory so that an impregnated catalyst could have a completely dry appearance or a sticky snow-like appearance. However, in no instances is there are any free flowing liquid present.
Furthermore, the impregnation technique may encompass all available methods besides incipient wetness, such as precipitation, impregnation from slurry with surplus liquid, chemical vapour deposition etc. It is well known that the impregnation method may influence the dispersion of the active metal (cobalt) and hence the catalytic activity, but as the Fischer-Tropsch reaction is believed to be non-structure sensitive, dispersion should not significantly influence the selectivity. The impregnated catalyst is dried, typically at 80-120° C., to remove water from the catalyst pores, and then calcined at typically 200-450° C., e.g. at 300° C. for 2-16 h.
A quantitative analysis of the comparison between cobalt and nickel as the primary metal in the Fischer-Tropsch reaction has been performed by H. Shultz, Topics in Catalysis, Volume 26, 2003, pages 73-85. Evidently, nickel has a higher hydrogenation activity than cobalt.
Nickel as a promoter to cobalt has not been described previously to our knowledge, but in EP-B-1 058 580, the possibility of using nickel as a modifying component for the support has been disclosed for the supports alumina, titania or magnesia. It is stated that the modifying component is able to suppress the solubility of the catalyst support in aquous acid or neutral solutions when calcined up to 800° C. to form a spinel compound. For nickel as modifying agent this implies that the spinel NiAl2O4 is formed, thus giving a more inert surface of the support. However, no example of the effect of nickel as modifying agent has been given.
Further, in EP-B-0296726, formed alumina particles have been impregnated with a solution of nickel nitrate and then calcined at a temperature of about 1200° C. in order to form a nickel aluminate spinel phase that strengthens the particles. It is pointed out that the heat treatment is performed under oxidative conditions to prevent reduction to the metallic nickel state, and therefore Ni is not used as a promoter. Further, the material produced is not used as a support for a Fischer-Tropsch catalyst and no mention is given of cobalt as an active phase.
The main features of an FT-catalyst are its activity, selectivity and stability. The cost of the catalyst both in terms of production costs and raw material expenses also must be considered. The desired selectivity depends on the products of interest for a given project, but in the present context the focus will be on the C5+ selectivity that often is used as an indicator of the wax formation and subsequent potential for maximum diesel production by hydro-isomerization/cracking of the wax.
These properties are somewhat interconnected, e.g. a high activity can give the possibility to reduce the operating temperature, thereby increasing C5+ selectivity. A high stability over time means that the initial activity can be relaxed, e.g. by reducing the cobalt loading or the cobalt dispersion.